1. Field of the Invention
The present invention relates to an electromagnetic control valve for a variable capacity compressor and, more particularly, to an electromagnetic control valve for a variable capacity compressor employed in a cooling unit for vehicles.
2. Related Art
The inventors of the present invention have developed an electromagnetic control valve for a variable capacity compressor employed in a cooling unit for vehicles, which is disclosed in Japanese Patent Application No. 8-109797.
FIG. 4 is a schematic view of a capacity control mechanism in a variable capacity compressor in which the above electromagnetic control valve is employed.
A compressor 20 is provided with an electromagnetic control valve 1-5 for capacity control at its attachment cavity portion 40. A plurality of cylinders 25 are provided inside the head portion leading to the crankcase 21 of the compressor 20. A piston 26 is slidably provided for each of the cylinders 25. A driving shaft 27 is rotatably provided between the crankcase 21 and the head portion 41. The driving shaft 27 is driven by an engine (not shown) with assistance of a belt 35 and a pulley 34 provided at the outer end.
The driving shaft 27 is provided with a known wobble plate 29 for angle variation. The wobble plate 29 is connected to the pistons 26 via piston rods 24. The inclined wobble plate 29 is rotated by the driving shaft 27 to reciprocate the piston rods 24 and the pistons 26. Thus, the attachment angle of the wobble plate 29 can be automatically adjusted depending on the difference between the control chamber pressure Pc inside the crankcase 21 and the suction side pressure Ps inside the cylinders 25. Accordingly, the stroke width of the pistons can be varied depending on the inclination of the wobble plate 29.
Each of the cylinders 25 has a suction inlet S and a discharge outlet D, and is connected, via passages d and s, to a condenser 31, an evaporator 32, and an expansion valve 33, which constitute the refrigerating cycle. The electromagnetic control valve 1-5 is led to a control chamber C inside the crankcase 21 via a refrigerant passage 36, to the suction inlet S via a refrigerant passage 37, and to the discharge outlet D via a refrigerant passage 38.
FIG. 5 is a sectional view of the electromagnetic control valve of the prior art when not energized.
The electromagnetic control valve 1-5 includes: (a) a valve member 8a which is provided in a valve chamber 18 formed by an upper main body 7 and a valve main body 17, and which repeatedly comes into contact with and depart from a valve seat 17d formed in a valve opening 17k situated on the way to a Pc pressure introducing opening 17h leading to the crank chamber C in the crankcase 21; (b) a Pd pressure introducing opening 17i which is formed on the opposite side of the Pc pressure introducing opening 17h from the valve chamber 18, and which leads to the discharge-side refrigerant passage 38 of the compressor 20; (c) a valve guide 17e extending from the Pd pressure introducing opening 17i to the Pc pressure introducing opening 17h; (d) a valve stem 8b which is integrally formed with the valve member 8a, and which is movable in the vertical direction inside the valve guide 17e; (e) a pushing unit 9 for pushing the valve member 8a in the valve opening direction; (f) a plunger 5 situated above the upper main body 7 which is vertically movable by virtue of the suction power of an electromagnetic coil 15, and which pushes the valve member 8a in the valve closing direction via a connecting rod 6 which is vertically movable in a through opening 7b of the upper main body 7; (g) a pressure equalizing opening 17q formed through the valve main body 17 and extending from the valve chamber 18 to the Pd pressure introducing opening 17i; (h) pressure bellows 11 formed below the Pd pressure introducing opening 17i and situated in a space 17g which communicates with the suction inlet S of the compressor 20; and (i) a corrective pin 10 which comes into point-contact with the lower surface of the valve stem 8 at its upper end and with the pressure bellows 11 at its lower end, and which is movable in a guide hole 17j extending from the Pd pressure introducing opening 17i to the space 17g.
With the suction pressure Ps of the piston cylinders 25 in FIG. 5, the pressure bellows 11 control the opening of the valve amber 8a, which is situated in the refrigerant passage extending from the refrigerant passage (a discharge pressure supply passage) 38 to the crankcase 21.
A casing 2, a coil guide 3, the plunger 5, the connecting rod 6, the upper main body 7, the electromagnetic coil 15, and a plunger tube 16, integrally constitute an electromagnetic actuator. The closing of the valve member is controlled depending on the current supplied to the electromagnetic coil by the electromagnetic actuator.
With the above electromagnetic control valve, however, there has been a problem that the control stability is often lowered. The contact surfaces between the plunger and the upper main body are a conic surface having uniform vertical angles and a plane vertical to the valve stem, respectively. If the plunger deviates from the center of the suction portion of the upper main body, the horizontal suction force toward the conic surface suction portion is great in the case where the plunger is situated close to the upper main body, and small in the case where the plunger is far from the upper main body. Because of the unbalance of the suction force, the plunger deviates further from the center, and is pressed by the plunger tube due to the horizontal suction force when moving. The friction resistance caused here appears as hysteresis, which lowers the control stability of the valve.
Since the magnetic force loss is great between the plunger and the coil guide, it has been necessary to use a coil large enough to obtain the coil suction force required.